1. Field of The Invention
The invention pertains to the field of optical source equipment and related methods for use in fiber optic communications. More specifically, the optical source equipment includes an optical source bank that is used, for example, in testing optical amplifiers and wavelength division multiplexing (WDM) systems.
2. Statement of the Problem
Rapid advances in WDM or dense wavelength multiplexing (DWDM) provide cost-effective increases in the capacity of fiber-optic data transmission systems through the use of multiple wavelengths of light. DWDM is a higher-capacity version of WDM. WDM systems support the multiplexing of up to four channels or wavelengths on a single fiber. Commercially available DWDM systems support up to 40 wavelengths or channels, and this capacity is steadily increasing. Data transmission capacity is also increased by time division multiplexing (TDM) rates in which a plurality of separate data signals are transmitted on the same line by interleaving a piece of each signal one after another in time. Maximum transmission capacity is determined as a trade-off between the DWDM channel count and the maximum supported TE)M switching rate. For example, a system operating on 40 channels at OC-48 with TDM might operate at a net throughput of about 100 Gbps. Future systems operating on the OC-192 protocol at 40 channels might have a net throughput of 400 Gbps, and a future system operating on 100 channels might have a net throughput of one terabit per second.
While these future systems are not yet practical, continuing advances in DWDM and TDM technology are expected to expand maximum net throughput rates over the next several years.
The advances in net throughput rates require corresponding advances in fiber-optic system components, which drive advances in test components, test equipment and signal amplifiers. For example, erbium-doped fiber amplifiers (EDFA) are used in DWDM systems to eliminate or minimize the use of regenerative repeaters, and can be used as in-line repeating amplifiers, transmitter booster amplifiers, and receiver pre-amplifiers. EDFA devices comprise a section of glass fiber, which is doped with erbium. The erbium-doped fiber amplifies laser light transmitted through the doped section of fiber. EDFA technology has been used, by way of example, to support a mix of four 2.5 Gbps digital video streams in delivery of an 80 channel AM cable television network signal over a 100 km distance using one EDFA at the output node and one at midspan.
Optical test equipment for use in testing system components for fiber-optic transmissions is continually outdated in the face of rapid capacity advances. Traditional DWDM test equipment uses an eight or sixteen channel multiplexer where, for example, eight channels may be allocated to a mainframe. Attempts to provide additional sources, e.g., more than 40 laser diodes that are each linked with a corresponding channel, source modulation electronics, attenuator, polarization control, and error injection devices produce unwieldy agglomerated test systems that are connected with a patchwork of optical cables. The test systems grow to occupy large amounts of space, and test measurement errors may be induced, for example, by movements in the optical cables that interconnect the respective devices. Depreciation and use of optical test equipment may comprise a substantial percentage, e.g., twenty or thirty percent, of the total manufacturing costs of optical system components.
A problematic side-effect of continually increasing capacity in optical data transmission systems is that increased amounts of space are required to accommodate the associated increases in volume of test equipment. For example, such equipment is regularly supported by racks. Each rack, according to industry standards, typically has dimensions of approximately seventy-two inches in height, twenty-seven inches in depth, and nineteen inches in width, which corresponds to a rack volume of about 21.4 cubic feet.
To date, no manufacturer has been able to offer an optical source array in the confines of a single integrated box containing more than about seventeen laser sources within the box. Within boxes formed to meet these restrictions the number of laser sources are primarily limited by the size of the electronic and optical assemblies required, and by heat dissipation requirements.
Many commercial systems do not use all of the available rack space because, for example, the housing may be shallower or shorter than the corresponding depth or height of the rack. One typical system housing a total of eight laser source cards has dimensions of five and one-quarter inches in height, seventeen and three-quarters inches in width, and seventeen inches of depth, for a total volume of 0.92 cubic feet. The eight laser sources or channels in a single box of this volume produces a density of approximately 8.7 laser sources per cubic foot inside the box, and approximately 1.5 laser sources per vertical inch in the rack. These numbers represent about the maximum density that can be obtained within a single box due to the size of the electronic and optical assemblies required, and to excessive heat dissipation from electro-optical components in the box. The addition of special cooling systems other than blowers for air convection systems is practically not done because it adds significant cost and maintenance complexity to these systems.